Environmental impact and survival of halibut larvae during first feeding
European Network for the Dissemination of Aquaculture
RTD Information (Q5CA-2000-30105) and previously FAIR-3837, Aquaflow ref. :
TL2003-194
Fiskey Ltd. is among the largest producers of halibut
fry in the world. From a 1996 production of 35.000 halibut fry, it has
developed to produce 440.000 fry in 2002 and 750.000 in 2003. The progress
can (to a large extent) be attributed to new research and improved methods,
which have been applied in order to decrease the bacterial load in the first
feeding phase.
The microbial flora is one of the factors that affect
the degree of normality, growth and survival during the first feeding period
of halibut larvae in intensive farming. The main goal of the project was to
reduce the bacterial load of the larvae and the surrounding environment and,
at the same time, to keep the use of disinfectants and chemicals at a
minimum without adverse effects on growth and survival of the larvae. The
exact effects of the various disinfectants and chemicals on the dominating
bacterial flora were examined with the best possible larval survival rates
as a goal. The results indicate that the bacterial load and/or composition
affect the growth and survival of halibut larvae more than has been
previously recognised. Furthermore, recent findings indicate that it is
possible to reduce the bacterial load considerably by using inorganic
substances instead of the conventional algae used for shading of the
environment during the first feeding. The possibility of using inorganic
substances (clay) along with algae or to replace the algae was examined and
the methods were compared with respect to the bacterial load and diversity
as well as the metamorphosis and survival rates of the larvae. The results
showed that the inorganic substance does not affect the total survival rate
of larvae and Fiskey Ltd. currently uses the inorganic substance
exclusively. This has lead to an increased safety and enhanced economic
feasibility and is considered an important step towards reduced production
costs in intensive farming of halibut larvae.
The results also show a clear correlation between the
average number of bacteria and the average percentage of gaping larvae at
the end of the yolk sack period. Gaping is a well-known problem where the
mouth is locked in an open position. The larvae are not able to feed and die
shortly after the content of the yolk sack is finished and they have to
start feeding on live food (Artemia).
Furthermore, the results indicate that oscillation
and unstability in the quality of the Artemia affects the growth and
survival of larvae to a greater extent than was previously thought
(different enrichments of Artemia in addition to the fact that
packing and processing seem to vary between different Artemia
producers). The results of the
project have lead to improvements of methods used in the treatment and
cultivation of the Artemia and
this is believed to have been the main reason for the increased number of
larvae produced during the last two periods (late 2002 and early 2003). The
growth and survival rate of the larvae has improved and there is a reason to
believe that this discovery will result in an even better and more stable
production of marine larvae during the first feeding period. The problem of
poor growth of environmental strains of bacteria on culture media in the
laboratory still persists and the next step will be to examine the total
bacterial flora and changes in the bacterial flora by mapping the genetic
material (PCR methods and DNA analysis).
The results also give a good reason to hope that
“good” bacteria (probiotics), along with environmentally friendly
disinfectants and chemicals, can be used to control both the number and the
composition of the bacterial flora in the farming units. The use of a
certain blend of probiotic bacteria gave promising results and there is a
reason to hope that use of probiotic bacteria could enhance the survival of
halibut larvae during the first feeding period.
For more information, contact:
Rannveig Björnsdóttir
The Icelandic Fisheries Laboratories and University of Akureyri, Glerargata
36
IS-602 Akureyri
Iceland
Tel: (354) 895 2176 - Fax: (354) 462 5216
E-mail: rannveig@rf.is